The need for scalable synthesis of bulk and thin-film selenide semiconductor materials for photovoltaic and other applications creates a growing demand for low-cost reactive selenium sources. One example of such sources is aqueous solutions containing Se2− or poly-selenide ions, which can be used directly for synthesis of metal selenide materials by reacting said solutions with metal salts or other compounds.
While selenium is well-known to easily dissolve in alkali hydroxides and different methods of its reduction into solution have been proposed (see, for example, U.S. Pat. No. 3,390,090 issued to Taylor et al., entitled “Metallic Selenides and Tellurides and Process for Making Same”), the introduction of metal counter ions, particularly alkalis may compromise the purity of the obtained semiconductor product. Ammonium counter-ions are preferred for stabilizing the reactive aqueous selenium solutions as they allow reaching high selenium concentrations (above 30%) and do not introduce metal impurities in the targeted semiconductor product.
Elemental selenium is one of the lowest-cost selenium sources for preparing ammonium selenide solutions. U.S. patent application Ser. No. 13/207,248 filed by Mitzi et al., entitled “Process for the Preparation of Elemental Chalcogen Solutions and Method of Employing Said Solutions in Preparation of Kesterite Films” describe the use of ammonia borane as a highly efficient reducing agent that was successfully applied for selenium solution preparation. However, challenges such as vigorous dissolution reaction, creating a potential for H2Se release and the high price of ammonia borane need to be addressed in order to reduce the cost of its large-scale application.
Aluminum selenide was proposed as another convenient source for preparing ammonium selenide solutions (see for example U.S. Pat. No. 3,306,701 issued to Thomson et al., entitled “Preparation of Selenides and Tellurides”) and was used for different metal selenide synthesis. The method can be summarized in two reactions:2Al+3Se→Al2Se3  (1)Al2Se3+6NH4OH→3(NH4)2Se+2Al(OH)3  (2)
However, the synthesis of aluminum selenide proceeds through the highly exothermic reaction between elemental aluminum and selenium (Equation 1), requiring heavy-duty synthesis equipment operating in an inert atmosphere, resulting in an unusually high price for this reagent. The reaction of aluminum selenide with ammonium hydroxide (Equation 2) is also exothermic, creating a potential for H2Se release.
While engineering improvements of methods found in the above-described conventional processes may reduce their cost and make them practical for large-scale selenium solution manufacturing, no convenient method has yet been found for the direct formation of metal-free reactive selenium solutions from elemental selenium.
Thus, improved techniques for the formation of metal-free reactive selenium solutions from elemental selenium would be desirable.